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Upper limits Warming ocean waters are pushing some fish living in the Tasman Sea beyond their comfort zone, say Australian scientists.

While a slight increase of water temperature can boost fish growth and reproduction living in cooler waters, laboratory studies have shown that fish stop growing and reproducing, and can eventually die, if temperatures rise too much.

Now research appearing in today's Nature Climate Science journal, by a group of scientists from the CSIRO and University of Tasmania, has shown that warming waters are beginning to push wild populations of banded morwongs off the coast of New Zealand beyond their physiological limits.

"This is the first time we've been able to show … the stresses are actually happening in the wild and … there's a physiological cost associated with [climate change]," says study co-author Dr Ron Thresher from the CSIRO's Climate Adaptation Flagship.

Red morwongs, or red moki (Cheilodactylus spectabilis), inhabit temperate reefs in waters between 10 and 50 metres deep. Living for up to 100 years, they tend to stay in the same territory throughout their life despite changes to their environment.

In the last 60 years, surface water temperatures in the Tasman Sea have risen more than 2°C. This temperature rise is caused by globally increasing sea surface temperatures and the movement of the warm East Australian Current further south.

The researchers compared the growth of fish living in five sites. This included the most northerly and warmest off the north coast of New Zealand, the most southerly and coldest off southeastern Tasmania.

To determine growth rates they analysed the fishes' otoliths, bony structures that fish use for orientation and detection of movement. Just like tree rings, the bony structures show incremental changes in fish growth over long periods of time.

They then compared the growth rates of the fish to the annual mean temperature change recorded at each of the five sites since the turn of last century.

Boom and bust

While fish were flourishing and growing in size in the cooler waters of the coast of Australia, which is periodically affected by the EAC, the researchers found that fish living off the coast of New Zealand were decreasing in size. Here, annual average water temperatures exceed 17°C and can reach between 18°C and 19°C during the spawning season.

"As things are getting warmer, the fish [in the cooler sites] are doing a little better. Providing there's food in the environment, they can tolerate a slight acceleration of their physiology," says Thresher.

"But what we've seen in these northern-most populations though is that we're past the point where a little bit of warming is a good thing."

"Their bodies are still speeding up because of the increasing temperatures, but now they're speeding up to the point where they can't tolerate it any more. They can't get enough energy, they can't breathe fast enough, and that's what's costing them growth rates," he says.

To test what they were seeing in the wild, the researchers conducted an additional study to identify the effect of increased water temperatures on spawning behaviour.

Results of the preliminary study indicate that once water temperatures reaches 16°C and above, the fish consume 44 per cent more oxygen, can't sustain the swimming speed needed to spawn, and go into anaerobic stress.

Study co-author Dr Jeremy Lyle from the University of Tasmania's Institute of Marine and Antarctic Studies says warming oceans have implications for fish reproduction and long term productivity of fisheries, especially along the northern margin the fish's range.

"We won't see fish die because of water temperatures," he says, "but the population in those areas will become less productive and really very dependent upon young fish moving into those areas and replacing them.

"If that's not occurring then what we might see is a contraction in the range of the fish at the warm end [of the distribution]."